Alkanes + Haloalkanes Flashcards
What are alkanes?
Saturated hydrocarbons
General formula alkanes
CnH2n+2
General formula cycloalkanes
CnH2n
What is petroleum/crude oil?
A mixture of hydrocarbons, mainly alkanes. Separated by fractional distillation
How does fractional distillation work?
- crude oil vaporised at 350°C
- in fractionating column rises up trays
- largest don’t vaporise, run to bottom- residue
- negative temperature gradient
- alkanes have different chain lengths, different boiling points, condense at diff temperature and drawn off at diff levels
Fractional distillation works because?
- more carbons in formula: greater intermolecular van der Waals’ forces
- greater if= more energy required to separate molecule= higher bp
- larger alkanes= higher bp
What is cracking
Breaking long chain alkanes into smaller hydrocarbons. Involves breaking C-C bonds
Why is cracking needed?
- Shorter chain (light) fractions e.g petrol, naphtha are in higher demand- more valuable
- excess larger hydrocarbons e.g bitumen cracked int9 more valuable shorter chain
What is a fraction
A group of similar length hydrocarbon chain which have similar boiling points that are collected in fraction of same temperature
Thermal cracking
- high temp: 450-900°C
- high pressure
- no catalyst
- produces lots of straight chain alkanes + alkenes (raw materials for chemical industry)
Catalytic cracking
- high temp: 450°C
- moderate pressure
- zeolite catalyst
- high proportion branched alkanes +alkenes, cyclic + mainly aromatic hydrocarbons +motor fuels
Why do alkanes make great fuels?
Burning a small amount releases huge amounts of energy. However, produces lots of pollutants
Complete combustion
Oxidise alkanes + other hydrocarbons with plenty of oxygen- CO2 and H20
C3H8(g) + 5O2(g) —> 3CO2(g) + 4H2O(g)
Incomplete combustion
- limited (not enough) oxygen
- produces carbon monoxide (CO) instead of or as well as CO2.
- soot (C) can be formed
Catalytic converter
Fitted to exhaust system of car to remove pollutants (unburnt hydrocarbons and oxides of nitrogen)
How does burning fossil fuels contribute global warming
- produces CO2: greenhouse gas
- absorb infrared, emit some of absorbed energy back to earth= greenhouse effect
- increase CO2 in atmosphere makes earth warmer
Pollutants made by internal combustion engine
- unburnt hydrocarbons
- oxides of nitrogen (NOx)- due to high pressure + temp cause nitrogen + O2 to react
- these react in presence sunlight-form ground level ozone (O3), component smog
- catalytic converters remove unburnt hydrocarbons and oxides of nitrogen
How is acid rain caused
- combustion of hydrocarbons containing sulfur leads to sulfur dioxide- air pollution
- in atmosphere dissolve in moisture, convert into sulfuric acid- acid rain
How can sulfur dioxide be removed
Powdered calcium carbonate/CaO mix with H2O= alkaline slurry.
Flue gas mix with alkaline slurry- acidic sulfur dioxide react to form harmless salt
What is a free radical
A particle with an unpaired electron
How are free radicals formed
Covalent bond is split equally, giving one electron to each atom (homolytic fission)
Substitution reaction
One atom in a molecule is replaced by another atom or group of atoms
Free radical substitution
Involves breaking a carbon- hydrogen bond in alkanes
Conditions needed for free radical substitution
-ultraviolet light: high energy to break strong covalent bonds (photodissociation)
-excess methane to reduce further substitution
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